Microstructural evolution in infiltration‐growth processed MgB2 bulk superconductors

Ashutosh G Bhagurkar,Hari B Nadendla,David A Cardwell,John H Durrell,Akiyasu Yamamoto,Anthony R Dennis,Talal A Aljohani

doi:10.1111/jace.14792

Abstract

AbstractThe study reports phase and microstructural evolution in MgB2 bulk superconductors fabricated by an infiltration and growth (IG) process. Three distinct stages, (1) intermediate boride formation, (2) bulk liquid Mg infiltration, and (3) MgB2 layer formation, were identified in IG process after detailed examination of series of samples prepared with varied heating conditions. The intermediate phase Mg2B25, isomorphous to β‐boron, was detected prior to MgB2 phase formation in stage (1). Due to volume expansion involved in stage 1, cracks formed in the β‐boron particles and propagated radially inwards during stage 3. The growing MgB2 particles sintered simultaneously with formation of grain boundaries during the process, as evidenced by the measured hardness and critical current density in these samples. From our observations, we estimate the total time needed for complete transformation to MgB2.

Highlights

Superconducting MgB2 is an exciting candidate for practical applications due to a relatively high Tc (39 K), low raw material cost, low density (2.6 g/cm3), ease of fabrication, and the absence of any deleterious effect on critical current arising from grain boundaries.[1,2] Infiltration and growth (IG) is a promising route for the fabrication of MgB2 bulk superconductors, typically involving impregnation of liquid magnesium, Mg(l), into a porous solid boron, B(s), preform
We divide the overall growth of superconducting MgB2 in the IG process into three stages: 3.1 | Stage 1: Initiation of cracks in the boron particle: Mg2B25 phase formation process
Given that X-ray diffraction (XRD) did not detect any traces of MgO (Figure 2), all the gain in weight of the sample can be attributed to liquid Mg only

Summary

Introduction

Superconducting MgB2 is an exciting candidate for practical applications due to a relatively high Tc (39 K), low raw material cost, low density (2.6 g/cm3), ease of fabrication, and the absence of any deleterious effect on critical current arising from grain boundaries.[1,2] Infiltration and growth (IG) is a promising route for the fabrication of MgB2 bulk superconductors, typically involving impregnation of liquid magnesium, Mg(l), into a porous solid boron, B(s), preform. The relative density of IG processed MgB2 is significantly higher (>90%) than samples fabricated using conventional sintering techniques, with process requiring no high-pressure apparatus.[3,4] near-net complex shapes can be fabricated with infiltration route that are not achievable by conventional sintering methods.[5,6]. DeFouw et al.[9] proposed a diffusion based model for MgB2 growth in boron fibers. They observed radial and circumferential growth of

Methods

Results

Discussion

Conclusion